The impact of acute malaria infection on the level of spontaneous apoptosis, i.e., the percentage of apoptotic cells detectable in lymphocytes cultured without any exogenous stimulus for 3 days in vitro, was evaluated. Quantitation of apoptosis was performed by staining of lymphocyte nuclei with propidium iodide and analysis of the fluorescence by cytometry. The mean apoptosis of 23 HIV-negative patients (15 Africans and 8 Europeans) determined during a confirmed Plasmodium falciparum attack was 27.2% (95% confidence interval (CI) = 23.5-30.7%) i.e., 2.2 times the mean level found in 49 controls (12.4%, CI = 11.1-13.6). These controls included age- and sex-matched Africans (n = 37) and Europeans (n = 12) differing only by their previous level of exposure to P. falciparum. Naive (European) as well as previously exposed (African) subjects showed dramatically elevated levels of spontaneous apoptosis during the malaria attack (mean = 22.5%, CI = 20.7-24.4 for Europeans; mean = 29.7%, CI = 24.6-34.7 for Africans). Such unusually raised levels were observed for at least 1.5 months and were probably detectable for longer periods as suggested by the fact that the mean level of spontaneous apoptosis in healthy Africans was basically higher (13.8%, CI = 12.5-15) than the one found in healthy Europeans (8.2%, CI = 6.3-10.1) (P = 0.0001). Selective immunomagnetic cell isolations carried out immediately before apoptosis quantitation showed that this process affected not only the alpha beta T cells (CD4+ T cells as well as CD8+ T cells) but also the gamma delta T cells and the B-lymphocyte subset.(ABSTRACT TRUNCATED AT 250 WORDS)